192 
THE PLANT WORLD. 
3. Poisoning .—This may be from gases in the air, which when 
absorbed by the leaves kill or injure their tissues. This is not 
likely to occur in the country, but it is common in and near man¬ 
ufacturing cities, especially so near smelting works, coke ovens, 
and like establishments, which emit chemical fumes in consid¬ 
erable volumes. A much more common source of poisoning is 
from the presence of harmful substances in the soil. Most trees 
cannot endure the presence of appreciable quantities of common 
salt, and when this occurs in the soil in sufficient amount they 
are slowly poisoned. So too the presence of “ alkali ” is harm¬ 
ful, as indeed is the case with many other soil constituents. It 
must be borne in mind that lime, magnesia, soda, iron, etc., very 
dilute solutions of which plants need or at least are tolerant of, 
when in strong solutions become fatally poisonous. In some cases 
the soil may be helped by the application of some corrective, but 
for the most part it will pay the tree grower best to make selec¬ 
tion of a soil which does not contain any of these substances in 
poisonous quantities. 
4. Wounds .—Attention has been called repeatedly to the in¬ 
jurious effects of wounds on trees; in fact they are comparable 
in their harmfulness to open wounds in animals. And just as 
in animals a wound is a place through which harmful bacteria 
may gain entrance to the body, so a wound in a tree allows 
various kinds of injurious organisms to enter the inner tissues. 
Every wound on a tree, from whatever cause, is a danger, and 
should be cared for, if the life of the tree is to be indefinitely pro¬ 
longed. The most general cause of wounds is the wind, which 
breaks or twists the branches and often splits them badly. In 
the dense forests trees are rarely injured in this way, as they pro¬ 
tect one another, but every isolated tree is especially subject to 
the power of the wind. Orchard trees are usually planted so far 
apart that they are exposed to almost the full fury of the wind, 
and after every storm many trees show many ragged wounds 
through which fungi of various kinds may enter. 
Another fruitful source of wounds is the gnawing of the bark 
by rabbits and other animals. These wounds are not as deep 
as those due to winds, and yet they are often even more harmful, 
